Mechanisms for creating heat for transfer to other relatively small items, such as automotive external rear view mirrors, are well known in the art. Traditionally, these have employed either resistance circuits to create heat along with a heat transfer plate affixed to the item to be heated. In some instances, light bulbs or lamps have supplied the resistance, and the heat generated by the current through the resistance of the lamp have been used to create heat for transfer to the item to be heated.
Heat transfer mechanisms of the type described above have been described, for instance, in German patent applications Nos. DE-OS 2151856 and in German Patent No. DE-OS 2724018 which describe the use of light bulbs located inside a automotive rear view mirror casing. In DE-OS 2151856, the light bulb is located adjacent the back of the mirror and a conductive plate is situated between the bulb and the mirror to provide a heatable surface through which heat generated by the bulb is transferred to the mirror. In DE-OS 2724018, several such light bulbs are employed.
The light bulbs of the prior art transfer heat primarily by radiation, which tends to be unreliable and spotty. The bulbs tend to draw substantial power, are subject to destruction by shock, and the levels of heat output of the bulb tend to change over time. Still further, the size of the bulb normally required limits its applicability to many devices requiring heating as too often, little space is available in which to mount the bulb. In external rear view mirrors for automotive uses, space is further limited in those mirror assemblies which contain mechanisms for adjusting the position of the face of the mirror, which take up substantial room within the body of the mirror. As one can appreciate, bulb replacement can and often would be a significant problem.
German Patent DE-OS 2724018 is directed to the use of several such bulbs for heating and in particular is directed to the use of a spring mounted mirror adapted for easy removal to permit replacement of the bulbs.
Both of the mirror assemblies described in the German patents discussed above provide heating primarily by irradiation rather than by conduction. In addition, both mirror assemblies described in these patents place the bulbs in positions dictated by the shape of the mirror, and the amount of heat supplied to the mirror is not easily subject to regulation.
In the automotive arts, the literature relating to automotive electronics does not contain material directed to the heating of external rear view mirrors which is either reliable or automatically controls the level of heat to which the mirror is subjected.